It is known to employ laser light sources in multi-spot laser plotters in order to illuminate a line in an optical image plane, thereby exposing simultaneously more than one spot and increasing the plot speed without exceeding the mechanical speed limitation. Thus, for example, U.S. Pat. No. 4,577,932 (Gelbart) describes a multi-spot light modulator using a laser diode in which a single light pulse from the laser diode generates a multi-spot image of a data pattern. Such a system is inherently faster than single-spot modulators since it records a plurality of bits at the same time.
In such systems, the width of the illuminated line determines the resolution of the laser plotter and the optics must therefore be designed so as to conform to the required resolution.
Furthermore, in order to produce a uniform image, the illuminated line must likewise be uniform. The requirements of tightly specified width and uniformity along the illuminated line are especially important when employing laser plotters to plot features of a very accurate dimension and position on a film.
In practical systems, it is difficult to realise these requirements for a variety of reasons. Effects such as concentric rings, aperture diffraction and optical imperfection all militate against the requirement to produce a uniform light distribution along the illuminated line. Optical imperfection is particularly severe in laser diodes having a large Numerical Aperture of the order of 0.4.
The laser source itself must be finely controlled in order to produce an illuminating line of constant width and uniformity with a specified optical system. In practice, ostensibly identical laser diodes having the same manufacturer and model number have a divergence spread which varies from diode to diode. This causes size variation of the width and length of the illuminating line and, in order to produce a line of constant, minimal width, the optical system would require customizing, by providing a custom designed collimating lens, for each laser diode in a batch. This, of course, is clearly impractical.
In order to optimize the optical performance at the illuminated area whilst, at the same time, maximizing the depth of focus in accordance with the basic laws of diffraction, the illuminating beam should be collimated and its waist should correspond to that for a source having a Gaussian shaped energy distribution along its width at any cross-section along its length. For such a source, the waist band is the area of minimum divergence.
The astigmatism in the laser diode source causes the beam to diverge from an apparently different position perpendicularly to the beam directions. The astigmatism itself and its variation from one diode to another affects the position and shape of the resulting line image.